1. Field of the Invention
This invention relates to electrical devices comprising PTC materials.
2. Background of the Invention
There are a number of known materials whose resistivity increases sharply with temperature over a relatively small temperature range. Such materials are said to be "PTC materials" or to "exhibit PTC behavior" , PTC being an abbreviation of "positive temperature coefficient". For many purposes, it is preferred that a PTC material should exhibit an R.sub.14 value of at least 2.5 and/or an R.sub.100 value of at least 10, and particularly preferred that it should have an R.sub.30 value of at least 6, where R.sub.14 is the ratio of the resistivities at the end and the beginning of a 14.degree. C. range, R.sub.100 is the ratio of the resistivities at the end and the beginning of a 100.degree. C. range, and R.sub.30 is the ratio of the resistivities at the end and the beginning of a 30.degree. C. range. Many PTC materials show increases in resistivity which are very much greater than these minimum values. A plot of the log of the resistance of a PTC element (i.e. an element composed of a PTC composition) against temperature will often show a sharp change in slope over a part of the temperature range in which the composition has an R.sub.100 value of at least 10. The term "switching temperature" (usually abbreviated T.sub.s) is used herein to denote the temperature at the intersection point of extensions of the substantially straight portions of such a plot which lie either side of the portion showing the sharp change in slope. The term "peak resistivity" is used herein to denote the maximum resistivity which the composition exhibits above T.sub.s, and the term "peak temperature" is used to denote the temperature at which the composition has its peak resistivity.
PTC elements have proved particularly useful as components of self-regulating heaters and of circuit protection devices. The PTC materials which have been used or proposed for use in such electrical devices are certain ceramics and certain conductive polymers, the term "conductive polymer" being used herein to denote a composition which comprises an organic polymer (this term being used to include polysiloxanes) and, dispersed or otherwise distributed in the organic polymer, a particulate conductive filler. Suitable ceramic materials include doped barium titanates, and suitable conductive polymers include crystalline polymers having carbon black dispersed therein. PTC ceramics generally exhibit a sharp change in resistivity at the Curie point of the material, and PTC conductive polymers generally exhibit a sharp change in resistivity over a temperature range just below the crystalline melting point of the polymeric matrix. The PTC ceramics which are used in commercial practice generally show a sharper rate of increase in resistivity than do the PTC conductive polymers. PTC ceramics generally have a resistivity of at least 30 ohm-cm at 23.degree. C., whereas PTC conductive polymers can have a lower resistivity at 23.degree. C., e.g. down to about 1 ohm-cm or lower. PTC ceramics tend to crack and thus to fail suddenly if exposed to excessive electrical stress, whereas PTC conductive polymers tend to degrade relatively slowly.
Documents which disclose circuit protection devices comprising PTC conductive polymers include the trade pamphlets published by Raychem Corporation in January 1987 and entitled "A General Approach to Circuit Design with PolySwitch Devices", "Protection of Subscriber Line Interface Circuits with PolySwitch Devices", "Protection of PBX and Key Telephone Systems with PolySwitch Devices", "Protection of Telecommunications Networks with PolySwitch Devices", "Protection of Loudspeakers with PolySwitch Devices", and "Protection of Batteries with PolySwitch Devices". ("PolySwitch" is a registered trademark of Raychem Corporation.) The disclosure of each of these trade pamphlets is incorporated herein by reference.
The term "hold current" (or "pass current") is used to denote the maximum steady current which can be passed through a PTC circuit protection device without causing it to trip (i.e. be converted into a high temperature, high resistance state such that the circuit current is reduced to a very low level). The hold current of a device depends upon the rate at which heat is lost from the device; for example, the higher the ambient temperature, the higher the hold current. It is known to connect a plurality of substantially identical devices in parallel to provide a PTC protection assembly having a hold current which is substantially equal to the sum of the hold currents of the individual devices. The performance characteristics of a PTC circuit protection device depend importantly on the voltage which is dropped across it in the tripped state; the higher the voltage, the greater the danger that the device will be damaged and will thus fail to provide the desired protection and/or will fail in a hazardous way, e.g. will explode or burn. As is apparent from the patents and applications incorporated herein by reference, much effort has been devoted to increasing the voltage which can safely be dropped over PTC conductive polymer circuit protection devices. In general, the greater the distance between the electrodes, and the greater the extent of the crosslinking of the conductive polymer, the higher the voltage which can be employed. While there are available protection devices which can safely handle a voltage of about 600 volts RMS, protection against higher voltages remains a problem. Another unsolved problem is the provision of devices which will protect against voltages that can be handled by existing devices, but which are easier to manufacture than existing devices (e.g. require less or no crosslinking) and/or which have a more convenient shape (the shape often being largely determined by the configuration and separation of the electrodes), either for installation or in use (e.g. on a printed circuit board or in other situations where space is at a premium) and/or for thermal balance considerations.